German Patent Publication DE-PS 2,558,765 (Gageur), published on Mar. 24, 1977, discloses a tentering frame for the continuous treatment of web type sheet material. Two guide rails carrying tentering clamps are provided in positions alongside the edges of the web or sheet material to be stretched. Each guide rail comprises individual spring steel tapes arranged on edge so that the tapes extend with their width perpendicularly to the plane of the material being treated. The tapes are clamped together to form the guide rails for the tentering clamps. The tentering clamps are equipped with pairs of guide rollers for the horizontal guiding and with support rollers for the vertical support and guiding.
Ideally, the guide rails should guide the tentering clamps, or rather the rollers carrying the tentering clamps without play. However, in conventional guide rails substantial play may be caused by various factors, such as manufacturing tolerances in the tentering clamps, varying on-center spacings between the vertically oriented axes of the guide rollers, manufacturing tolerances in the thickness of the rolled spring steel tapes and other unavoidable influences. As a result, such play may occur between the guide rollers and the guide rail. Such play has the disadvantage that the guide rollers of the tentering clamps do not always contact the guide rail as they travel along the guide rail, whereby these rollers do not rotate constantly. This does not apply to the upper support rollers which, due to gravity, remain in contact with the respective guide rail.
Depending on the loading and guiding of the tentering clamps during the film stretching operation, either a first set of guide rollers or a second set of guide rollers or guide roller pairs come into contact with the guide rail in alternating fashion. This means that the rotation of the guide rollers may stop after the guide rollers lift off the respective guide rail. Thus, when contact is again established between the guide rollers and the guide rail the rotation of the guide rollers must be accelerated in a very short time to the travel speed of the tentering chain carrying the tentering clamps or rather accelerated to the circumferential velocity of the rollers which rotate in response to the just mentioned travel speed.
Such repeated accelerations and decelerations cause dynamic load changes in or on the tentering chain and on the guide rollers with disadvantageous effects on both especially with regard to an increased wear and tear. The just mentioned dynamic load changes may in a most disadvantageous situation cause the guide rollers to perform sliding motions on the guide rails rather than rolling motions, whereby the outer guide roller surface ring and the respective outer tape of the guide rail are subjected to even greater wear and tear. Depending on the internal friction of the guide roller bearing and of the sealing system of the guide roller bearing damages can occur due to grinding contacts between elements moving relative to each other. Such damage is especially effective on the outer ring surface of the guide roller. Further, damaged guide rollers tend to mechanically damage and impose increased wear on the outer spring steel tapes while simultaneously wearing down the outer guide roller rings. Even before these mechanical damages occur, the guide roller bearing is exposed to heat generated by friction due to the repeated acceleration and deceleration causing the above-mentioned sliding movements. Such friction heat occurs in addition to the relatively high temperatures in the heating zone of the tentering machine.
German Patent Publication DE 195 15 036 A1 (Lindner et al.), published on Oct. 31, 1996, discloses guide rails for tentering clamps of a tentering machine in which an improved guiding of the tentering clamps is achieved, thereby increasing the functional operability and the useful life of the tentering machine. In the Lindner et al. disclosure the spacing between the horizontal guide surfaces of the guide rail and the spacing between the vertical guide surfaces of the guide rail are maintained variable in limited ranges. In other words, the horizontal guide rail thickness and the vertical guide rail width are variable in these limited ranges. For this purpose at least one biasing and/or damping device is provided, whereby at least one of the just mentioned guide surfaces are maintained in a variable position and/or are supported by a damping device. Independently of the construction of the guide rail as a compact single piece or as a combination of differently constructed guide rail elements, the integration of the above-mentioned position variable biasing and/or damping devices into the guide rail involves a complicated structure. As a result, relatively high manufacturing and assembly expenses are involved.